Signal transmission systems

Abstract

A signal transmission system impresses phase-modulated signals on to a multimode optical fibre waveguide 1. In order to overcome the effects of fading caused by small random changes in the parameters of the fibre the signals from different modes are all combined to derive a resulting signal which is detected in a common detector 23. The signals may be combined by a vibrating mirror 21 which sweeps the emergent beam across detector 23. Alternatively the tip of fibre 1 may be vibrated to achieve a similar effect.

We claim: 
     
       1. A signal transmission system comprising: means for propagating coherent light along a multimode optical fibre waveguide;   modulation means for impressing phase-modulation signals in said waveguide and allowing the impressed signals in different modes to interfere in the waveguide so that an amplitude modulated signal is generated which is subject to random fading across the waveguide;   means provided at the output of the waveguide for inducing intermodal phase-modulation of the amplitude modulated signals from different parts of the cross-section of the waveguide to derive a single optical modulated signal in which the random fading is neutralized; and   detector means comprising a single photodetector to which the said single optical modulated signal is applied.   
     
     
       2. The system as claimed in claim 1 in which the said inducing means comprise a mechanical vibrator coupled to the waveguide. 
     
     
       3. A signal transmission system comprising: means for propagating coherent light along a multimode optical fibre waveguide;   modulation means for impressing phase-modulation signals in different modes in said waveguide, said impressed signals in different modes interfering in the waveguide to produce an amplitude modulated optical signal, said amplitude modulated optical signal tending to be subject to random fading at respective cross-sectional portions of the waveguide;   a single photodetector disposed in optical receiving relationship with end of the waveguide; and   means for effecting a relative motion between component portions of said amplitude modulated signal from respective cross-sectional portions of the waveguide and said detector to combine said component portions into a single amplitude modulated optical signal not subject to fading and applying said single amplitude modulated optical signal to said single photodetector.   
     
     
       4. The system as claimed in claim 3 in which said means for effecting a relative motion comprises scanning means for repetitively scanning across the beam emerging from the end of the waveguide so as to obtain a succession of signals from a plurality of positions spaced apart across the beam. 
     
     
       5. The system as claimed in claim 4 in which the scanning means comprises a vibrating mirror. 
     
     
       6. The system as claimed in claim 4 in which the scanning means comprises an elastic medium through which the beam emerging from the waveguide is passed and means for subjecting the said medium to an acoustic wave. 
     
     
       7. The system as claimed in claim 6 in which said acoustic wave repetitively varies the refractive index of the said medium. 
     
     
       8. The system as claimed in claim 6 in which the acoustic wave sets up a diffraction pattern in the said medium to cause the beam to be repetitively deflected between two positions. 
     
     
       9. The system as claimed in claim 4 in which the scanning means comprises means for causing the tip of the waveguide to vibrate. 
     
     
       10. The system as claimed in claim 9 in which the said scanning means comprises a piezo-electric crystal secured to the end of the waveguide and means for applying an oscillatory electrical signal to the crystal. 
     
     
       11. The system as claimed in claim 10 in which the crystal is in the form of a cantilever beam clamped at a position in the waveguide spaced from the tip thereof. 
     
     
       12. The system of claim 4, 5, 6, 7, 8, 9, 10, 11, 2 or 1 in which the said modulation means functions to modulate at two different frequencies in succession so that the resulting signal represents binary digital information.